Parametric stereo or multi-channel audio coding as described e.g. in C. Faller and F. Baumgarte, “Efficient representation of spatial audio using perceptual parametrization,” in Proc. IEEE Workshop on Appl. of Sig. Proc. to Audio and Acoust., October 2001, pp. 199-202, uses spatial cues to synthesize down-mix—usually mono or stereo—audio signals to signals with more channels. Usually, the down-mix audio signals result from a superposition of a plurality of audio channel signals of a multi-channel audio signal, e.g. of a stereo audio signal. These less channels are waveform coded and side information, i.e. the spatial cues, relating to the original signal channel relations is added to the coded audio channels. The decoder uses this side information to re-generate the original number of audio channels based on the decoded waveform coded audio channels.
A basic parametric stereo coder may use inter-channel level differences (ILD) as a cue needed for generating the stereo signal from the mono down-mix audio signal. More sophisticated coders may also use the inter-channel coherence (ICC), which may represent a degree of similarity between the audio channel signals, i.e. audio channels. Furthermore, when coding binaural stereo signals e.g. for 3D audio or headphone based surround rendering, also an inter-channel phase difference (IPD) may play a role to reproduce phase/delay differences between the channels.
The synthesis of ICC cues may be relevant for most audio and music contents to re-generate ambience, stereo reverb, source width, and other perceptions related to spatial impression as described in J. Blauert, Spatial Hearing: The Psychophysics of Human Sound Localization, The MIT Press, Cambridge, Mass., USA, 1997. Coherence synthesis may be implemented by using de-correlators in frequency domain as described in E. Schuijers, W. Oomen, B. den Brinker, and J. Breebaart, “Advances in parametric coding for high-quality audio,” in Preprint 114th Conv. Aud. Eng. Soc., March 2003. However, the known synthesis approaches for synthesizing multi-channel audio signals may suffer from an increased complexity.